1. Field of the Invention
The present invention relates to a light source device including an electric discharge tube that emits light by discharging electricity in a gas. In particular, the present invention relates to a light source device including an electric discharge tube comprising a cold-cathode tube using mercury as a discharge gas.
2. Description of the Related Art
A liquid crystal display device includes a backlight using an electric discharge tube comprising a cold-cathode tube, as a light source, for example. In the electric discharge tube, mercury is used as a light-emitting material. The mercury exists in the electric discharge tube in both liquid and gaseous states. The amount of the filled mercury is far (about 1,000 times) larger than the amount of vapor mercury that provides the maximum light emitting efficiency. In such an electric discharge tube, the maximum light emitting efficiency can be obtained when the electric discharge tube is at a predetermined temperature. However, if a large amount of electric power is supplied to the electric discharge tube, the temperature in the electric discharge tube rises so as to increase the mercury vapor pressure too much and to reduce the light emitting efficiency.
In Japanese Unexamined Patent Publication (Kokai) No. 2003-100130,that is the prior application of the present application, in order to prevent this phenomenon from reducing the efficiency, a technique has been proposed to provide a heat conducting path (a heat conducting material) that is in contact with the electric discharge tube so that heat is transferred from the electric discharge tube via the heat conducting path to cool the electric discharge tube. By providing such a heat conducting path, the liquid mercury is concentrated at the cooling part of the electric discharge tube that is in contact with the heat conducting path so that the mercury vapor pressure in the electric discharge tubes corresponds to the temperature of the cooled part of the electric discharge tube, which results in the maximum light emitting efficiency.
It is desirable that the liquid mercury is situated at the cooling part of the electric discharge tube that is in contact with the heat conducting path in a concentrated manner. However, the liquid mercury is distributed randomly in the tube before use and it concentrates at the cooling part gradually during use. Therefore, it is desirable to perform a procedure to concentrate or collect the liquid mercury at the cooling part in advance and before use.
In the prior art, there have been the following problems:
(a) if the process to concentrate the liquid mercury at specific positions is needed when the electric discharge tubes are manufactured, the cost may be increased;
(b) when droplets of the liquid mercury are displaced from the cooling part, due to impact, and the like, the light emitting efficiency may be reduced;
(c) conventionally, a heat conducting silicon rubber material (heat conductivity 1 to 2 W/K/m) or an aluminum plate is used as the heat conducting path. In both materials, the heat conductivity hardly depends on the temperature. Therefore, because the magnitude of temperature rise of the electric discharge tube, that is exposed to the ambient temperature, is always proportional to the heating value of the electric discharge tube, as the ambient temperature rises, the surface temperature of the electric discharge tubes varies proportionally and, as a result, the temperature inside the electric discharge tube cannot be maintained at an optimal value; and
(d) if the heat conducting path is large and dark in color, it may cast a shadow when it is incorporated as the backlight.